Security alarm on solar cells. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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For a burglar alarm device, light can serve as a good source of energy and provide it with a detector circuit located at some distance from it.

In fact, such a burglar alarm completely feeds itself.

Principle of operation of the device

You need to start with a light source. A beam of light is directed along a doorway, window or room, forming a security zone. At the receiving end, the solar cell detects the presence of a light beam and converts it into electricity.

The solar cell plays a major role in the operation of the device; it not only detects the light, but also powers the signaling circuit itself.

The whole secret lies in the choice of an alarm scheme that has been specifically designed to ensure minimum power consumption. Due to this characteristic of the circuit, the output signal of the solar cell is simultaneously used as useful information about the light beam and for powering the entire device.

Schematic diagram

The signaling scheme can be divided into three parts. Let's start with the photoelectric converter.

In fact, the solar cell, which has been discussed so far, meant a solar battery of five cells connected in series. The total output voltage of the battery is 1,6 V at a current of about 1 mA, depending on the actual illumination of the cells.

First of all, the solar panel must provide power to the alarm circuit. This is achieved by charging a small rechargeable nickel-cadmium battery. The charge circuit contains a solar cell, a diode D1 and a battery. When a "guard" light beam falls on the surface of the solar battery, the battery is charged by the current flowing through the diode D1.

We know from the previous chapter that the battery will drop the charging voltage down to about 1,35V. From this point of view, the battery can actually be thought of as a zener diode. Given the voltage drop of 0,3 V across diode D1, the voltage of the solar array itself stabilizes at 1,65 V.

The current from the solar battery also flows through the resistances R1 and R2. The magnitude of this current is less than 250 μA, while most of the current goes to charge the battery.

Resistors and R2 are an important part of the detection circuit. Consider everything in order (Fig. 1).

When current flows through R1 and R2, voltage division occurs. The resistances of resistors R1 and R2 are chosen so that when the solar cells are illuminated, the voltage drop across the resistor R1 is only about 0,21 V. This voltage adds up to the voltage drop across the diode D1 (0,3 V), resulting in a potential difference between the base and the emitter transistor Q1 is 0,51 V.

Since Q1 is a silicon transistor with a minimum bias voltage of 0,7 V, the base voltage is too low to turn on the transistor. When the solar cell is illuminated with light, the transistor is locked and no current flows through it.

Ris.1

However, when the light beam is interrupted, the current from the photoelectric converter stops, therefore, no current flows through the resistor R1. The current also stops through the diode D1.

What happens is that R1 becomes a high-impedance source, D1 is a reverse-biased diode (due to the loss of voltage from the solar cell), and current flows through R2 and the base-emitter junction of transistor Q1. Now the collector current will appear.

Collector current is supplied to IC1 (alarm generator). This design uses this particular microcircuit, since it operates at an extremely low supply voltage and consumes very little current. At a supply voltage of 1,5 V (typical for signaling), the LM3909 chip goes into an unstable state and, therefore, will be in the generation mode. The values ​​\u5b\u6bof the components R1, RXNUMX and CXNUMX determine the frequency of generation.

The LM3909 also contains a power amplification output stage. By connecting an acoustic transducer (loudspeaker) between the generator output (pin 2) and the positive terminal of the battery, a loud, clearly distinguishable signal can be heard when the generator is running.

When the light beam is interrupted, the detector circuit is immediately triggered and an audible signal sounds. When the light beam is restored, transistor Q1 turns off and generation stops. Thus, the circuit plays the role of a bell ringing when a door or gate is opened.

Fixing an alarm

If automatic circuit recovery is undesirable, for example in an intruder alarm system, a latching circuit is introduced into the base unit. These are basically the elements of the R3, Q2 and R4 circuit, however, the whole trick of the fixation circuit is determined by the LM3909 chip.

A 5 ohm resistor is connected inside the microcircuit between pins 6 and 12. As long as there is no voltage applied to positive terminal 5, it will also be absent at terminal 6. This is the state of the schema prior to commit.

When the light beam is interrupted, transistor Q1 turns on and energizes pin 5, starting the oscillator. Potential also appears at pin 6. If the "latching S1" switch is on, then the voltage from pin 6 through the resistor R4 is supplied to the base of the transistor Q2. Current begins to flow through transistor Q2 and resistor R3, further increasing the current already flowing through the base of transistor Q1.

Even if voltage is supplied from the solar cell again, the flow path of the current generated by the solar cells changes significantly. As a result, the resistance of the resistor is no longer less than the resistance of the resistor R2 and the voltage drop across R1 increases. The effective resistance of R2, R3 and Q2 becomes small compared to R1, and the solar cells are not able to bring the transistor Q1 out of saturation. Thus, the alarm signal will be given even when the light beam is restored. It can only be switched off with switch S1.

Security alarm design

The basis of the design is a battery, made up of five miniature solar cells connected in series and outwardly resembling a tiled roof. It is clear that quite small elements can be used, since they require a minimum current. It is not easy to make such a battery without sufficient knowledge of the technique of cutting elements and the corresponding devices for this. It is strongly recommended that you purchase a pre-assembled battery listed in the parts list.

To increase the range of the burglar alarm, solar cells are equipped with a parabolic mirror. The mirror collects rays of light from a large space and focuses them on the elements. A portable flashlight was used for this purpose, and you can do the same.

You need to choose a flashlight with the largest lens aperture you can find - this is important. Then disassemble the reflector assembly and remove the bulb. In this design, the lens not only concentrates light rays, but also protects the mirror reflector from mechanical damage and moisture.

Now the solar battery is glued from the inside to the transparent protective lens in its center, while the back side of the battery should be facing the lens. The lens is set in place so that the solar array is located against the hole from the light bulb. Two conductors from the battery are passed through this hole, and then the reflector is fixed.

Of course, the battery blocks a significant part of the transparent lens, and because of this it is necessary to choose the largest possible reflector. You can also reduce the size of individual solar cells and reduce the size of the battery.

Since the outputs of serial solar panels are not color-coded, you must determine their polarity yourself. The wire soldered to the front surface of the lower element has a negative polarity and is attached to the body. The other wire soldered to the back of the top element has a positive polarity. A similar distribution of the polarity of the electrical leads is typical for pn-junction solar cells, in which the upper illuminated layer is n-type; for solar cells made from basic n-type silicon, the top p-type layer and the polarity of the leads will be the opposite of that indicated in the text.

The detector and generator units of the device are placed on the printed circuit board shown in fig. 2, and the placement of parts on it - in Fig. 3.

Ris.2

Ris.3

All parts are soldered to the board, except for the solar panel. If you connect it, the alarm will go off. If desired, you can install a switch in series with the battery, which allows you to turn off the alarm when not in use.

The printed circuit board is installed in the flashlight compartment, usually intended for batteries. It is necessary to place the board so that the acoustic transducer communicates with the outside, otherwise its shrill sound will be muffled. In addition, a hole is drilled in the housing for the "fixation" switch. It is necessary to fix the conductors coming from the solar cells and carefully assemble the flashlight, this time soldering the battery to the circuit. The security device is ready to go.

If the installation is correct, the system will give a shrill alarm. To "calm" it, it is necessary to turn off the triggering fixation and illuminate the surface of the solar cells. This is easy to do: before the system is installed in the place designated for it, it is placed under a table lamp.

Installation of a security alarm

A typical scheme for installing a security device in a doorway is shown in fig. 4. It is fixed at a height of 60 cm, sufficient for most cases. A beam of light is directed so that it blocks the passage into the room.

Ris.4

Now you need to install an alarm device on the opposite side of the opening. It may be necessary to adjust the direction of the light beam so that it accurately hits the surface of the solar cells. This is easy to establish: when the beam is accurately aimed, the alarm will stop.

Any powerful flashlight can be used as a light source. For this purpose, the same flashlight was taken as for placing the alarm circuit. The battery was replaced with a 6-volt step-down transformer, with one end of the 6-volt winding connected to a light bulb and the other to the mains.

If you want to make the light beam invisible, you can use an infrared filter. Even red cellophane will make the beam less noticeable. Since a silicon solar cell has significant sensitivity in the red and infrared regions of the spectrum, the loss in sensitivity will be negligible. However, the attenuation introduced by the filter must be taken into account: the range of the system cannot be expected to remain the same.

Please note that if the filter covers the surface of the light emitter, it may heat up. The degree of heating depends on the type of filter and its transmission. High heat may cause fire.

Using an AC powered light source has the added benefit of signaling a power outage.

Author: Byers T.

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